The Second Atmospheric River Will Bring Far More Severe Impacts

In act one of this meteorological drama, yesterday and this morning brought heavy rain and flooding to many sections of western Washington.  Well-predicted days in advance.

But the weather prediction models indicated that this would be a two-act event, with the second act being far more serious.   The curtain is about to rise.

Take a look at the precipitation totals of the first act (last two days, below).  Some areas received 7-8 inches.

Many local rivers are now at flood stage, with some at record levels (black dots are record levels, blue are 90th percentile or great, with normal being 50th percentile)

The media is full of pictures of flooded roads, such as in the Snoqualmie Valley.  A once-in-1-3-year event.

But this is just the meteorological appetizer course.  The real action is about to begin:  a second, stronger event that will throw massive amounts of rain on saturated surfaces.

Consider the total precipitation from the UW ultra-high resolution modeling system for the 72 h start this morning at 4 AM (shown below).

Wow.  There are going to be some happy ducks.  Large areas will get 10 inches of rain more, particularly in the Olympics and northern/central Cascades.

Serious flooding is inevitable.   

How can I be so sure?  Because of the use of ensembles of many high-resolution forecasts.  If they are all on board with heavy rain, my confidence grows.

Let me demonstrate this to you.   Here is the ensemble of accumulated precipitation at Humptulips on the southwest side of the Olympics.  A huge amount of precipitation (about 6 inches), most over about 12h.   Not much variance in the forecasts.

Stampede Pass in the central Cascades (below)?  A similar soggy story.

Every reservoir in the region will reach normal or above normal levels.  Consider the Seattle Times' favorite drought reservoir system (the Yakima), which they featured in a recent story.

The Yakima River is now at flood stage (see below), and in a few days the entire system will be full to normal levels.